Weight reduction of a vehicle door enables fuel efficiency of a vehicle to be improved and further contributes to reduction of CO2 emission from the vehicle. Therefore, technological development of weight reduction of the vehicle door has been promoted. For example, JP 61-169316A and JP 2013-163441 (References 1 and 2) disclose a vehicle door that includes an outer panel formed of a synthetic resin having a lighter weight than metal.
According to References 1 and 2, the vehicle door includes an inner panel and a reinforcement member which are formed of metal and thus, it is not possible to sufficiently reduce weight of the vehicle door. In addition, in a case where the reinforcement member is formed of a resin, there is a concern that rigidity of the vehicle door is lowered.